1. Field of the Invention
The present invention relates to a safety apparatus for a vertical injection molding machine equipped with a clamping apparatus adapted to vertically move a movable mold by use of a drive unit that includes a rotation transmission mechanism using a transmission belt.
2. Description of the Related Art
Conventionally, there has been known a vertical injection molding machine equipped with a clamping apparatus adapted to vertically move a movable mold in order to carry out a clamping operation.
Such a vertical injection molding machine generally has the following structure. A stationary mold is attached onto a molding machine bed. A movable platen is fixed to the upper ends of a plurality of tie bars which are supported by the molding machine bed to be slidable in the vertical direction. A movable mold is attached to the lower surface of the movable platen, and an injection apparatus is disposed above the movable platen. Meanwhile, a clamping apparatus that utilizes the tie bars is disposed under the molding machine bed. The clamping apparatus includes a drive platen attached to the lower ends of the tie bars and a drive unit for vertically moving the drive platen. The drive unit comprises a toggle mechanism, a ball screw mechanism, a servomotor, and a rotation transmission mechanism. The toggle mechanism is disposed between the drive platen and the molding machine bed. The ball screw mechanism includes a nut attached to the toggle mechanism, and a screw that is rotatably attached to the center of the drive platen. The servomotor is attached to the drive platen and is equipped with a brake. The rotation transmission mechanism connects the servomotor and the ball screw mechanism.
By virtue of the above-described structure, when the servomotor is operated, rotation of the servomotor is transmitted to the screw of the ball screw mechanism via the rotation transmission mechanism, so that the nut is vertically displaced with respect to the screw. Consequently, the toggle mechanism expands and contracts, and the drive platen moves upward and downward, so that the movable mold opens and closes with respect to the stationary mold.
In such a vertical injection molding machine, a relatively heavy portion is moved vertically, in contrast with the case of an ordinary horizontal injection molding machine. Therefore, the drive unit utilizes a servomotor equipped with a brake, which prevents rotation of the screw of the ball screw mechanism when the movable mold is stopped at the open position. The screw of the ball screw mechanism would otherwise rotate due to a load that is imposed thereon in a reverse direction. However, when a transmission belt is used in the rotation transmission mechanism of the drive unit, a considerably strong tension partly stemming from clamping force is applied to the transmission belt, resulting in the following problems. The transmission belt is inevitably placed in a state in which the belt easily breaks. If the belt breaks when the mold is in the open position, heavy members, including the movable mold and the movable platen, fall by gravity; i.e., due to a load that is imposed on the ball screw mechanism in a reverse direction, so that safety cannot be ensured.
In order to solve this problem, as disclosed in Japanese Patent Application Laid-Open (kokai) No. 7(1995)-88922, there has been proposed a safety apparatus which detects breakage of a transmission belt by use of a transmission belt breakage detection switch, and which, upon detection of breakage, locks the screw of a ball screw mechanism by a brake unit attached to the ball screw mechanism to thereby prevent falling of a movable mold and other associated members.
However, the above-described safety device involves the following problems.
First, since the main function of the brake unit is braking, braking performance is insufficient for stopping or locking the screw quickly and reliably. Especially, when a brake unit attached to a clamping apparatus of a vertical injection molding machine and used in an ordinary molding cycle is also used for locking the screw, the above-described problem cannot be avoided, because the braking performance deteriorates quickly due to wearing of pads.
Second, a problem similar to the above-described problem also occurs when transmission becomes impossible due to establishment of a state in which a transmission belt is partially broken but is kept continuous by the remaining unbroken portion, or due to deterioration of the transmission surface (tooth face) of a transmission belt (timing belt). In such a case, the conventional detection means, which only detects presence/absence of the transmission belt by use of a detection switch, cannot detect abnormal conditions (defects) other than breakage of the transmission belt. The conventional detection means has insufficient performance for precisely and reliably detecting an abnormal condition of the transmission belt.